This invention relates to an optical communication set and, more particularly, to an optical communication set employing the same devices and equipments at the transmission and reception sides adapted, for example, for a communication by using temporarily laid optical fiber lines or for a test communication utilizing laid optical fiber cables.
An encountering speech test or an announce call order speech test is necessary in installation and maintenance of optical fiber used as transmission media of optical communication.
As shown in FIG. 1, assume that optical cables C.sub.A and C.sub.B are laid in adjacent blocks A and B and the optical fibers F.sub.1, F.sub.1 of the cables C.sub.A and C.sub.B are connected at the contacts in both blocks to each other. A laser light source LD is connected to the optical fiber F.sub.1 of the optical cable C.sub.A at the transmission terminal in the block A. Similarly, an optical power meter PM is connected to the optical fiber F.sub.1 of the optical cable C.sub.B at the reception terminal of the block B. The optical fibers F.sub.1, F.sub.1 of the cables C.sub.A and C.sub.B are connected so that the quantity of light from the source LD reaches the maximum value on the power meter PM.
In such a connecting work, at least three people must work at the transmission, reception terminals and connecting place, and they must be enabled to communicate.
Conventionally, these persons can communicate by connecting metallic line pairs (not shown) provided in the cables C.sub.A and C.sub.B to a telephone set. However, the metallic line pairs, which are not necessary except for the test speech, tend to be omitted from the optical cables from an economic viewpoint.
Thus, in case of other optical fibers in the cables C.sub.A and C.sub.B in this laying work such as, for example, as shown in FIG. 1, it is requested to communicate test speech or test data exchange by connecting optical speech transceiver T/R to the optical fiber F.sub.3.
A ping-pong transmission system is known as a transmission system for communicating in two ways through an optical line.
This system is in general a system for alternatively transmitting and receiving pieces of block information D.sub.A, D.sub.B compressed in time base at transmission and reception sides as shown in FIG. 2. This system is excellent at the point of eliminating the influence of near-end crosstalk. However, since the information to be transmitted is divided into blocks and transmitted, the longer the transmission line is, the more the delay time Td increases, and the more transmission interval Ti increases. Thus, this system has such a disadvantage that memory having large storage capacity is required. Further, this system has drawbacks that transmission equipment increases in size and becomes expensive.
A digital two-way transmission system employing the existing telephone line, i.e., two metallic lines, has been proposed.
The digital two-way transmission system has a pair of exclusive transmission lines at both transmission and reception sides. This system is set to such timing relation that pieces of information D.sub.A, D.sub.B to be transmitted in a burst state at a predetermined interval T.sub.1 are not superposed at the transmission and reception terminals.
However, the conventional technique is limited to the metallic line pair, and in order to take timing of pieces of information D.sub.A, D.sub.B to be transmitted, one channel unit (at telephone station side) is designated as master side, and the other is secured to a speech unit (subscriber's side) at the slave side. However, no actual technique for performing an optical communication system using the same optical communication equipments at both master and slave sides on both transmission and reception sides has been provided. Particularly, no technique for eliminating problems arising from the use of one optical line (not the metallic line pair), such as deterioration of information due to reflection occurring in the optical line or variation in transmission level due to optical lengths, has been invented.